Approximately 130,000 pacemakers were implanted in the United States in 1993, at a cost of well over $1 billion. Pacemaker recipients may have single chamber ventricular or dual chamber pacemakers implanted. Dual chamber pacemakers are more expensive and less long-lived, but more physiologic, than single chamber pacemakers. The purpose of this 5 year mode Selection Trial (MOST) is to determine whether dual chamber rate modulated pacing (DDDR) in patients with sick sinus syndrome: improves event-free survival; leads to superior quality of life and functional status; is more cost-effective than rate modulated single chamber (VVIR) pacing. The primary endpoint of MOST is a composite endpoint consisting of all cause mortality or stroke. The study is designed with a 90% power to detect a 25% difference between groups (p=0.05, 2 tailed). Secondary endpoints include health status and cost-effectiveness, total mortality, cardiovascular mortality, development of atrial fibrillation, development of pacemaker syndrome, and a combined clinical endpoint of non-fatal stroke, heart failure hospitalization, or death. Eligible, consenting patients with sick sinus syndrome will undergo DDDR pacemaker implantation and be randomly and noninvasively programmed to DDDR pacemaker implantation and be randomly and noninvasively programmed to DDDR or to VVIR. Enrollment will take place over 3 years, and follow-up will vary from 1.5 years to 4.5 years. Clinical and electrocardiographic data will be assessed by semi-annual visits, and quality of life will be assessed in a 1400 patient subset by yearly telephone interviews. Health care expenditures also will be assessed. Thus, this study will assess the patient benefits (clinical endpoints and health status), and societal benefits (cost effectiveness) of pacing mode selection in sinus node dysfunction.